74 INTRODUCTION. 



alluvial land. We have, moreover, distinct evidence of old 

 land-surfaces, both in the Coal-measures and in other cases 

 (as, for instance, in the well-known " dirt-bed " of the Pur- 

 beck series). When, for example, we find the erect stumps 

 of trees standing at right angles to the surrounding strata, 

 we know that the surface through which these send their 

 roots was at one time the surface of the dry land, or, in 

 other words, was an ancient soil (fig. 12). 



CONCLUSIONS AS TO CLIMATE. In many cases fossils 

 enable us to come to important conclusions as to the climate 

 of the period in which they lived, but only a few instances 

 of this can be here adduced. As fossils in the majority of 

 instances are the remains of marine animals, it is mostly the 

 temperature of the sea which can alone be determined in this 

 way ; and it is important to remember that, owing to the 

 existence of heated currents, the marine climate of a given 

 area does not necessarily imply a correspondingly warm 

 climate in the neighbouring land. Land-climates can only 

 be determined by the remains of land-animals or land-plants, 

 and these are comparatively rare as fossils. It is also im- 

 portant to remember that all conclusions on this head are 

 really based upon the present distribution of animal and 

 vegetable life on the globe, and are therefore liable to be 

 vitiated by the following considerations : 



a. Most fossils are extinct, and it is not certain that the 

 habits and requirements of any extinct animal were exactly 

 similar to, or even at all resembling, those of its nearest 

 living relative. 



&. When we get very far back in time, we meet with 

 groups of organisms so unlike anything we know at the 

 present day as to render all conjectures as to climate found- 

 ed upon their supposed habits more or less uncertain and 

 unsafe. 



c. In the case of marine animals, we are as yet very far 

 from knowing the exact limits of distribution of many species 

 within our present seas ; so that conclusions drawn from 

 living forms as to extinct species are apt to prove incorrect. 

 For instance, it has recently been shown that many shells 



